The present invention relates to the analysis of physiological electrograms, in particularly but not exclusively for identifying pathological cardiac conditions.
Previous research has shown that the risk of sudden death due to cardiac arrhythmias can be predicted by observing the shape of recorded endocardial electrograms in response to pacing.
The diagnostic change in electrograms consists of small deflections in the recorded electrogram following early stimulation of the heart. The heart is stimulated with apparatus that generates a stimulation sequence at one site in the heart and records electrograms from other sites within the heart.
The pacing sequence comprises of a number of stimuli at a constant rate, known as S1 stimuli. After a pre-set number of S1 stimuli an early stimulus is introduced known as the S2 stimulus or ‘extra-stimulus’. The sequence is repeated. Typically the interval between S1 and S2 stimuli is reduced on each occasion until the interval is so short that the heart is no longer able to respond to the S2 stimulus.
The interval between the S2 stimulus and the following S1 stimulus is the same as the S1-S1 interval.
The predictive method depends on demonstrating that the electrogram following an extra-stimulus becomes prolonged and contains more peaks. This effect in patients that are at high risk of sudden death becomes more pronounced as the interval between the S1 and S2 stimuli is reduced.
Each individual potential of the electrogram following an extrastimulus is identified together with its delay after the extra stimulus. These data can subsequently be analyzed to predict the risk of sudden cardiac death.
A long standing problem with this method has been the reliable detection of small individual potentials within the response to an extra stimulus. This stems from the presences of noise in the recorded electrical signals that may be created by other electrical equipment within a typical catheter laboratory. The electrical noise may vary widely between different laboratories. The problem is that of reliably distinguishing between potentials in the electrogram that are of physiological origin as opposed to spurious potentials caused by electrical interference.
GB2439562 describes a method of processing date from electrograms to reduce noise. The method comprises correlating an electrogram signal with several templates to produce a correlator output associated with each template. The electrogram signal may be passed through a high pass filter beforehand.
The correlator output from trace 1 is compared with the traces produced from the other templates. The selected trace that is considered most similar is used.
A fundamental problem is that any series of templates that purport to represent a physiological signal will be correlated, and therefore the results of each correlated trace will not be independent of each other. This creates considerable difficulties in to how to combine the various correlator outputs to give optimal signal detection and avoid spurious over detection and under detection of physiological potentials within the signal.
The noise can be reduced further by identifying the peak-to-peak amplitude of the correlated output within a period of the electrogram when no physiological signal is presumed to occur. This is used to create a threshold in which any peak having an amplitude below this threshold is considered to be noise. However, signals of physiological origin may have amplitudes which are close to the threshold, as a consequence is if the threshold is set too low, the physiological derived peaks will be detected but many other peaks will also be detected due to noise. Conversely, if the amplitude threshold is too high, physiologically important features of the signal may not be detected.